Light localization has been used effectively to enhance the performance of many devices that rely on an efficient interaction between light and matter. Light localization refers to an increase of the light intensity in a local region on the axis or longitudinal direction of the light propagation. Light localization can be achieved using Fabry-Perot type cavities, periodic gratings, photonic crystals, micro-resonators, periodic or non-periodic distribution of layers that alternate high and low index of refraction materials, and many other distributions of dielectric or metallic materials. In photovoltaic devices, photons (light) absorbed by the photovoltaic active material are converted to electron-hole pairs or charged carriers. An effective absorption of the light by such photovoltaic material can be achieved when the thickness of such layer is longer than the material photon absorption length in a broad range of the solar spectrum. This length varies from one photovoltaic material to another but, an effective absorption of photons in a broad range of the solar spectrum may require the use of 10s or 100s of microns of active material. Several drawbacks are linked to the use of such thick material layers as, an increase in material cost, an increase in electron-hole recombination due to the finite carrier drift or diffusion length, or a reduction in transparency for cells meant to be used as windows, for instance, in Building integrated photovoltaics (BIPV). Several techniques to increase light absorption in thin-film devices and methods of manufacturing the same have been disclosed in patents and journal publications.
U.S. Pat. No. 4,126,150 sets forth a transparent layer which thickness is adjusted to increase the solar radiation absorption efficiency.
J. Meier et al. Thin Solid Films 451-452 (2004) 518-524 report on the use of an anti-reflection multilayer design in order to couple more light inside an amorphous silicon p-i-n cell.
U.S. Pat. No. 4,442,310 discloses a spacer layer in between the back metal contact and the body of the active material to reduce the reflectivity of the photovoltaic cell in a particular wavelength range. A similar approached applied to dye sensitized solar cells and using a one-dimensional photonic crystal to achieve reflection was disclosed in US 2011/0030792 A1.
R. R. Lunt et al. Applied Physics Letters 98 (2011) Art. No. 113305 report on the use of distributed Bragg reflector mirror to increase reflectivity in the infrared which subsequently increases the efficiency of a low efficiency transparent organic solar cell.
In amorphous silicon cells, typically, an increase in the absorption light trapping is achieved by introducing a textured substrate and special back reflectors. This leads to a large suppression of losses due to optical reflection outside the cell or to light transmission to the back contact as described by Ruud E. I. Schropp and M. Zeman in “Amorphous and Microcrystalline Silicon Solar Cells,” published by Kluwer Academic Publishers (1998) p. 160-162. However, textured substrates or textured layers are a source of diffusion which in a transparent cell to be used for instance, in automobile windshields or in architectural installations would lead to a loss of clear vision.